This invention relates to the recovery of gold from ores and, more particularly, to a process for the recovery of gold from refractory carbonaceous ores.
Significant quantities of gold ore currently processed in North America include substantial portions of the sulfidic minerals arsenopyrite, pyrite and marcasite. The predominant sulfide minerals in the ore contain encapsulated gold. The gold in such ores is too fine to liberate economically merely by grinding. Leachants such as cyanide and thiosulfate cannot penetrate the metal sulfide particles. This problem has been overcome by pressure oxidation as disclosed in Thomas et al. U.S. Pat. No. 5,071,477. In such processes, the refractory ores are treated by pressure oxidation to oxidize sulfide sulfur before leaching. If the sulfide sulfur is not substantially oxidized, leaching is inhibited and gold remains locked in the sulfides. By treating the ore in an aqueous slurry at elevated temperature and oxygen pressure, the sulfur is oxidized and removed from the ore. Thereafter, the gold is readily leached with a leaching agent and acceptable yields result.
Pressure oxidation is typically performed by passage of ore slurry through a multi-compartmented autoclave to which an oxygen-containing gas is continuously supplied. Pressure oxidation typically occurs under acidic conditions, as oxidation of the sulfides in the ore produce sulfuric acid in the autoclave. For certain ores, the ore slurry may be treated with sulfuric acid prior to pressure oxidation to neutralize carbonates such that the slurry fed to the autoclave is acidic. Depending on the ore, however, pressure oxidation may be best effected under alkaline conditions, as in the process disclosed in Mason et al., U.S. Pat. No. 4,552,589.
Gold is typically recovered from pressure oxidized slurries using conventional cyanidation technology. The pH of the oxidized slurry is adjusted to between 10 and 11 with lime, and cyanide is added to solubilize the gold. Oxygen is dispersed through the slurry by agitation, and gold dissolves by the following reaction: EQU 4Au+O.sub.2 +8CN.sup.-- +H.sub.2 O.fwdarw.4Au(CN).sub.2.sup.-- +4OH.sup.--
In modern cyanidation circuits, the dissolved gold is typically adsorbed onto particles of activated carbon, either during the cyanide leach itself by carbon-in-leach (CIL) or following the leach by carbon-in-pulp (CIP). An alternate method of recovering gold from cyanide leach solutions is through zinc cementation and variations of the Merrill-Crowe process.
In addition to the locking of gold particles in sulfide minerals, a problem which must be addressed in the treatment of some ores is preg robbing. In carbonaceous ores, preg robbing occurs as active carbon indigenous to the ore has the ability to rob gold from the cyanide bearing leach solution, reducing recovery. Pressure oxidation can partially deactivate the indigenous carbon, but by itself is not sufficient for highly preg-robbing ores. To further reduce preg-robbing problems, blanking agents such as kerosene or sodium lauryl sulfate have been used to further deactivate carbon in the ore. Carbon-in-leach has been successful for mildly preg-robbing ores, as the activated carbon added to the slurry possesses adsorption kinetic characteristics superior to those of the indigenous carbon, allowing the gold to load onto the added carbon as soon as it is leached, before it can load onto the carbon in the ore. Carbon-in-leach alone, however, has not been completely successful in treating highly preg-robbing ores.
An additional problem in recovering gold from highly carbonaceous ores is that a significant quantity of the gold has been adsorbed onto carbon during formation of the mineral deposit. This gold will only become available to a lixiviant which can remove it from the carbon. The use of a cyanide lixiviant alone has not been entirely successful in leaching gold locked in carbonaceous material.
Ball et al., U.S. Pat. No. 4,902,345, disclose treating refractory carbonaceous and sulfidic ores by thiourea leaching in the presence of carbon rather than cyanide leaching. Kerley, Jr., U.S. Pat. Nos. 4,269,622 and 4,369,061, disclose using an ammonium thiosulfate leach solution containing copper to leach gold and silver from ores containing manganese. Perez et al., U.S. Pat. No. 4,654,078, disclose leaching gold and silver with a copper-ammonium thiosulfate lixiviant to produce a pregnant leach solution. Gold and silver are then precipitated onto a copper cement added to the pregnant leach solution. Wan et al., U.S. Pat. No. 5,354,359, disclose leaching gold from preg-robbing ores with a thiosulfate lixiviant followed by cementation or precipitation of the leached precious metal values. PCT application WO 91/11539 discloses recovering gold from a gold-loaded thiosulfate solution by adding cyanide to form a gold cyanide complex followed by adsorbing the gold cyanide complex onto a carbon or resin adsorbent.